Intervertebral implant

ABSTRACT

An intervertebral implant having a three-dimensional body ( 10 ) and a securing plate ( 1 ). The three-dimensional body ( 10 ) includes an upper side ( 1 ) and an underside ( 2 ) which are suitable for abutting the end plates of two adjacent vertebral bodies, a left side surface ( 3 ) and a right side surface ( 4 ), a front surface ( 5 ) and a rear surface ( 6 ), a horizontal middle plane ( 7 ) between the upper side ( 1 ) and the underside ( 2 ), and a vertical middle plane ( 12 ) extending from the front surface ( 5 ) to the rear surface ( 6 ). The three-dimensional body further includes a plurality of boreholes ( 9   a ) passing through the body ( 10 ), which are suitable for accommodating longitudinal fixation elements ( 20 ). The intervertebral implant also includes a front plate ( 8 ) displaceably disposed as an insert with the front side ( 5 ) of the three-dimensional body, the front plate ( 8 ) having a plurality of boreholes ( 9 ) in which the longitudinal fixation elements ( 20 ) can be anchored, and whose openings overlap with the openings of the boreholes of the three-dimensional body ( 10 ). A securing plate can be fastened essentially parallel to the front plate ( 8 ) at the three-dimensional body ( 10 ) in such a manner that the boreholes of the front plate ( 9 ) are covered at least partly by the securing plate ( 18 ). By virtue of the configuration of the intervertebral implant, a rigid, firm connection between the intervertebral implant and the longitudinal fixation elements used to fasten it, is possible.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of International Patent ApplicationNo. PCT/CH2003/000089, filed Feb. 6, 2003, the entire contents of whichis expressly incorporated herein by reference.

TECHNICAL FIELD

The present invention relates generally to intervertebral implants.

BACKGROUND OF THE INVENTION

GB-A-2 207 607 discloses an intervertebral implant, which has ahorseshoe-shaped configuration with a plurality of cylindrical holes.The holes are smooth on the inside and only have a stop for the heads ofthe bone screws, which are to be introduced therein. A disadvantage ofthis arrangement is that the fastening screws, introduced therein, canbe anchored only with their shaft in the bone. This does not result in arigid connection with the horseshoe-shaped intervertebral implant. Whenthe anchoring of the screw shaft in the bone is weakened, theintervertebral implant becomes movable with respect to the screw and thebone screws tend to migrate, endangering the blood vessels. Moreover,the loosening of the intervertebral implant can lead to apseudoarthrosis.

U.S. Patent Publication US-A 2000/0010511 (Michelson) discloses anintervertebral implant, which, at its front surface, has two boreholeswith an internal thread, into which bone screws with a threaded head canbe introduced. A disadvantage of this implant is that the bone screwscan become loose and are not secured against being screwed out orfalling out. A further disadvantage is that the bone screws are fastenedcompletely to the implant body itself and that therefore the latterexperiences a relatively large stress.

Screws which emerge at the anterior or anterolateral edge of thevertebral body because of loosening run the risk of injuring mainvessels such as the aorta and Vena calva, as well as supply vessels suchas lumbar arteries and veins. Injury to these main vessels may result ininternal bleeding possibly causing death within a very short time.Loosening of screws is more likely when they are not mounted angularlyfirmly.

SUMMARY OF THE INVENTION

The present invention is to provide a remedy for the above-discusseddisadvantages. The present invention is directed to an intervertebralimplant which can enter into a permanent, rigid connection with bonefixation means, so that, even if the bone structure is weakened, thereis no loosening between the intervertebral implant and the bone fixationmeans. Moreover, over a separately constructed front plate, there istension chording for the bone fixation elements, so that the implantbody experiences less stress, that is, superimposed tensions. Moreover,a securing plate enables all bone fixation elements to be securedsimultaneously.

The present invention accomplishes the objective set out above with anintervertebral implant, comprising a three-dimensional body having anupper side and an under side which are suitable for abutting the endplates of two adjacent vertebral bodies. The three-dimensional bodyfurther includes a left side surface and a right side surface, a frontsurface and a rear surface, a horizontal middle plane between the upperside and the under side, and a vertical middle plane extending from thefront surface to the rear surface. The three-dimensional body furthercomprising a plurality of boreholes, having openings at least at or nearthe front surface, passing there through and being suitable foraccommodating longitudinal fixation elements. The intervertebral implantfurther including a front plate displaceably disposed as an insert withthe front side of the three-dimensional body, where the front plateincludes a plurality of boreholes having openings and in which thelongitudinal fixation elements can be anchored, and whose openingsoverlap with the openings of the boreholes of the three-dimensionalbody. The intervertebral implant has a securing plate fastenedsubstantially parallel to the front plate in such a manner that theboreholes of the front plate are covered at least partly by the securingplate. An advantage achieved by the present invention, arisesessentially from the solid connection between the intervertebral implantand the longitudinal fixation elements, used to fasten it.

Compared to the two-part implants of the state of the art, for which afront plate is implanted in a separate step, the present invention hasthe advantage that the implantation of the intervertebral implant may becarried out in one step and, with that, can be carried out more easilyand more quickly. A further advantage is that the intervertebral implantis fixed as frontally as possible at the body of the vertebra. That is,at a place where good bone material usually is present. The result is ananterior movement limitation without a greater risk to the surroundingstructures. The load is still absorbed under compression by theintervertebral implant and not by the front plate or the fixation screws(longitudinal fixation elements).

A method for implanting an intervertebral implant of the presentinvention between two adjacent vertebral bodies includes introducing theintervertebral implant, having a three-dimensional body, a front plate,and one or more boreholes, between two adjacent vertebral bodies,attaching longitudinal fixation elements with heads through theboreholes into the vertebral bodies, and attaching a securing plate bymeans of a fastening agent over the heads of the longitudinal fixationelements to the front plate, such that the heads of the longitudinalfixation elements are captured between the front plate and the securingplate wherein the longitudinal fixation elements are secured againstbeing shifted relative to the intervertebral implant.

Other objectives and advantages in addition to those discussed abovewill become apparent to those skilled in the art during the course ofthe description of a preferred embodiment of the invention whichfollows. In the description, reference is made to accompanying drawings,which form a part thereof, and which illustrate an example of theinvention. Such example, however, is not exhaustive of the variousembodiments of the invention, and therefore, reference is made to theclaims that follow the description for determining the scope of theinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an exploded drawing of the intervertebral implant,

FIG. 2 shows a longitudinal fixation element in the form of a screw,

FIG. 3 shows an elevation of the intervertebral implant of FIG. 1,

FIG. 4 shows a side view of the intervertebral implant of FIG. 1,

FIG. 5 shows a three-dimensional detailed representation of the body ofthe intervertebral implant, which shows the connecting elements to thefront plate of FIG. 6,

FIG. 6 shows a three-dimensional detailed representation of the frontplate of the intervertebral implant and the connecting elements to thebody of FIG. 5 and

FIG. 7 shows a completely installed intervertebral implant with frontplate and securing plate.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The intervertebral implant, shown in FIG. 1-7, includes athree-dimensional body 10 in the form of a cage with an upper side 1 andan underside 2, which are suitable for abutting the end plates of twoadjacent vertebral bodies, a left side surface 3 and a right sidesurface 4, a front surface 5 and a back surface 6, a horizontal middleplane 7 located between the upper side 1 and the underside 2, a verticalmiddle plane 12 extending from the front surface 5 to the rear surface 6and four boreholes 9 a, which pass through the body 10 and are suitablefor accommodating longitudinal fixation elements 20. The body 10 may beconstructed as a hollow body, the mantle surfaces of which are providedwith perforations 19. The upper side 1 and/or under side 2 of theintervertebral implant may preferably be convex in shape, not planar. Aconvex shape to the upper side 1 and the underside 2 allows for animproved fit with the end plates of the adjacent vertebral bodies by theintervertebral implant. Further, the side surfaces 1-6 of theintervertebral implant may be essentially convex, as well.

As shown in FIG. 7, the upper side 1 and the underside 2 of thethree-dimensional body 10 are provided with structuring in the form ofteeth 30.

At the front surface of the three-dimensional body 10, a front plate 8may be mounted, which is disposed perpendicular to the horizontalcentral plane of the intervertebral implant and through which fourboreholes 9 pass and in which the longitudinal fixation elements 20 canbe anchored. The front plate 8, as shown in FIGS. 5 and 6, isconstructed as an insert for the three-dimensional body 10. Thethree-dimensional body 10 has a semicircular groove 27 extendingparallel to the vertical middle plane 12 at the transitions of the leftside surface 3 and the right side surface 4 (FIG. 5) to the frontsurface 5. Correspondingly, the front plate 8 has right and left (FIG.6) similarly extending and similarly dimensioned, semicircular rail 28.As a result, the front plate can be pushed and positioned easily withits two lateral rails 28 into the corresponding grooves 27 of the body10 during the production of the intervertebral implant.

In one embodiment, at least one of the boreholes 9 in the front plate isconstructed so that a longitudinal fixation element 20, accommodatedtherein, can be connected rigidly with the front plate. A rigidconnection may be accomplished, for example, owing to the fact that atleast one of the boreholes 9 of the front plate 8 has an internalthread. A corresponding longitudinal fixation element 20, bone screw,with a threaded end can then be screwed together rigidly with theimplant. In an alternative embodiment, the four boreholes 9 in the frontplate may have an internal thread 11, so that longitudinal fixationelements 20 can be connected rigidly with the front plate 8.

As discussed, the front plate 8 may be disposed, preferably verticallyto the horizontal central plane, so that it can be displaced verticallywith respect to the three-dimensional body 10. By these means, “stressshielding” (protection and neutralization of mechanical stresses) isattained, which permits the end plates to be adapted to theintervertebral implant during the healing process.

The intervertebral implant may have a securing plate 18, which can befastened by means of a screw connection parallel to the front plate 8 atthe front plate 8 in such a manner that the boreholes 9 of the frontplate 8 are partly covered by the securing plate 18. The securing plate18 may have a central borehole 17 provided, preferably, with an internalthread. Corresponding thereto, the front plate 8 has a central borehole15 for accommodating fastening means 16. Preferably, the centralborehole 15 has an internal thread 14 for accommodating a fasteningmeans 16 in the form of a screw. The securing plate 18 may also befastened by a bayonet catch or a click catch. By fastening the securingplate 18 to the front plate 8, the heads 21 of the longitudinal fixationelements 20 (discussed later) are contacted by the securing plate 18, sothat they are secured against being ejected or screwed out.

Preferably, the boreholes 9 a of the three-dimensional body 10 do notpass either through the left side surface 3 or the right side surface 4or completely through the front surface 5. The front surface 5,preferably, is also not crossed by the boreholes 9 a. Further, thehorizontal middle plane 7 is not pierced by the boreholes 9 a. Only theaxes 24 of the longitudinal fixation elements 20, introduced therein,intersect the horizontal middle plane 7 of the body 10. As seen from thefront surface 5, the boreholes of the three-dimensional body 10 and thefront plate diverge. The axes 24 of the boreholes of thethree-dimensional plate 10 and the front plate 8 enclose an angleranging from 20° to 60°, specifically from 36° to 48°, and morepreferably an angle β of 42° with the horizontal middle plane 7 (FIG. 4)and an angle α ranging from 10° to 45°, specifically from 27° to 33°,and more preferably an angle α of 30° with the vertical middle plane 12(FIG. 3). Thus, better access for introducing the screws is achieved.

In one embodiment, at least one of the boreholes 9 of the front plate 8may taper conically towards the underside 2, so that a bone screw, witha corresponding conical head, can be anchored rigidly therein. Theconical borehole preferably has a conical angle, which is smaller thanthe resulting frictional angle. Advisably, the conicity of the conicalborehole is 1:3.75 to 1:20.00 and preferably 1:5 to 1:15.

In another configuration, at least two of the boreholes 9 of the frontplate 8 extend parallel to each other. This makes insertion of theintervertebral implant easier. In another embodiment, at least two ofthe boreholes 9 of the front plate 8 diverge when viewed from the frontside. By these means, a region of the vertebral body, which has a betterbone quality than does the center of the vertebral body, is reached bythe bone screws.

To improve the anchoring of the bone screw in a plastic body of theintervertebral implant (discussed later), a metal sleeve with aninternal thread (not shown) may be inserted in the boreholes of thefront plate and three-dimensional body. The intervertebral implant mayalso consist only partially of an x-ray transparent plastic and, in theregion of the boreholes consist of a metal, such as titanium or atitanium alloy. Improved guidance and anchoring of the bone screws inthe intervertebral implant may be achieved. Further, the boreholes 9 mayhave a smooth internal wall, into which the threaded head of a metallic,longitudinal fixation element may cut or be molded.

Depending on circumstances, two, three, four or more longitudinalfixation elements may be connected rigidly with the intervertebralimplant. Preferably, at least one fixation element should pierce theupper side and at least one fixation element the underside of theintervertebral implant. The longitudinal fixation elements 20 may haveeither a smooth head, so that there will not be a rigid connection withthe implant or a threaded, conical or expendable end, so that there willbe a rigid connection with the implant. In both cases, however, thelongitudinal fixation elements 20 are secured by the securing plateagainst rotating out, being ejected out or falling out at a later time.

The longitudinal fixation elements 20 are preferably constructed as bonescrews. As shown in FIG. 2, the longitudinal fixation elements 20,introduced into the boreholes 9, have a head 21, a tip 22, a shaft 23and an axis 24. The head 21 may preferably be provided with an externalthread 25, which corresponds to the internal thread 11 of the borehole9, so that the heads 21 can be anchored in the boreholes 9 in a rigidmanner. The shaft 23 may be provided with a thread 26, which isself-drilling and self-cutting. The load thread angle of the thread 26has a range of between 11° to 14°, preferably between 12° and 13°, andmore preferably a load thread angle of 115°. The pitch angle of thethread may have a range of between 6° and 10°, preferably between 7° and9°, and more preferably have a pitch angle of 8°. The special pitchangle produces a self-retardation in the thread, thus ensuring that thebone screw will not automatically become loose.

In the case of a second, possibly rigid type of connection, alongitudinal fixation element 20, bone screw, may preferably be used,the head of which tapers conically towards the shaft, the conicity ofthe head corresponding to the conicity of the borehole of theintervertebral implant. The longitudinal fixation elements may also beconstructed as threadless cylindrical pins, which are provided with adrilling tip, preferably in the form of a trocar. A further variationconsist therein that the longitudinal fixation elements are constructedas spiral springs. Finally, the longitudinal fixation elements may alsobe constructed as single-vaned or multi-vaned spiral blades.

As shown in FIG. 7, two longitudinal fixation elements 20 pierce theupper side 1 and two longitudinal fixation elements 20 pierce theunderside 2 of the body 10, thereby anchoring the intervertebral implantto the adjacent vertebral bodies.

The intervertebral implant may be produced from any material which iscompatible with the body. Preferably, the three-dimensional body 10 mayconsist of a body-compatible plastic which has not been reinforced andwhich may be transparent to x-rays. The advantage over fiber-reinforcedplastics, which are already known in implant technology, is that noreinforcing fibers are exposed. Such exposure may be disadvantageousclinically. In such a three-dimensional body 10 constructed of a plasticthat has not been reinforced, the use bone screws may be preferable. Asdiscussed previously, the external thread of the bone screw(s) may havea load thread angle range of 11° to 14°, and preferably between 12° to13°. A comparatively slight inclination of the load flank brings about ahigh clamping force. As a result, radial expansion and the danger offorming cracks in the plastic are reduced. Furthermore, the externalthread of the bone screw(s) may preferably have a pitch angle between 6°and 10° and preferably between 7° and 9°.

The front plate 8 may be made from materials different than thethree-dimensional body 10. The front plate 8 is preferably made from ametallic material. Titanium or titanium alloys are particularly suitableas metallic materials. The complete tension chord arrangement (frontplate and screws) may also be made from implant steel or highly alloyedmetallic materials, such as CoCrMo or CoCrMoC. The advantage of titaniumlies in that there is good tissue compatibility and the good ingrowingbehavior of bones. The advantage of highly alloyed metallic materialslies in their high-strength values, which permit filigree constructions.

A brief description of a surgical procedure follows in order to explainthe invention further.

The intervertebral implant, in the form of a three-dimensional body 10,is introduced between two adjacent vertebral bodies by means of asuitable instrument. Longitudinal fixation elements 20, in the form ofbone screws, securing the three-dimensional body 10 are screwed/insertedby means of a suitable aiming device through the boreholes 9 of thefront plate 8 into the vertebral bodies. The front plate 8 may bedisplaced vertically with respect to the three-dimensional body 10, suchthat the openings of the boreholes 9 a of the three-dimensional plate 10and the boreholes 9 of the front plate 8 overlap, to obtain stressshielding. The securing plate 18 is fastened by means of the fasteningagent 16 in the form of a screw over the heads 21 of the longitudinalfixation elements 20 at the front plate 8, so that the heads 21 of thelongitudinal fixation elements 20 and, with that, the screws themselves,are captured between the front plate 8 and the securing plate 18 andsecured against being shifted relative to the three-dimensional body 10(for example, by falling out or by turning out). The fastening agent 16,in the form of a screw, preferably is provided with a thread, which isdistinguished by a large self-retardation.

1. A bone stabilizing system for stabilizing first and second vertebraein a spinal column comprising: a plurality of bone screws having athreaded shaft and a head, the shaft having a longitudinal central axis;a three dimensional body configured for insertion between the endplatesof vertebrae, the three dimensional body having an upper surface forabutting the endplate of the first vertebrae, a lower surface forabutting the endplate of the second vertebrae, and a front surface; anda front plate having a top surface and a bottom surface, the platecoupleable to the body such that the bottom surface is adjacent to andat least partially overlies the front surface, the plate having aplurality of screw holes for receiving the shafts of the screws, thescrew holes extending through the plate and having an axis, at least twoscrew holes extending through the plate at an angle relative to thebottom surface of the plate so that the axis of the respective screwhole ranges from about 30° to about 70°.
 2. The system of claim 1further comprising a securing plate operatively connectable to the frontplate in a manner to at least partially contain at least one of theheads of the screws receivable in one of the angled screw holes betweenthe front plate and the securing plate.
 3. The system of claim 1 furthercomprising a securing plate operatively connectable to the front platein a manner to at least partially cover at least two of the angled screwholes in the front plate.
 4. The system of claim 1 wherein the frontplate and at least one of the screw holes in the front plate aredesigned and configured such that one of the screws inserted into theangled screw hole pierces one of the upper or lower surfaces of thethree-dimensional body when the plate is coupled to thethree-dimensional body.
 5. The system of claim 4, wherein a plurality ofthe angled screw holes in the front plate and the front plate aredesigned and configured such that one of the screws inserted into afirst angled screw hole pierces the upper surface of thethree-dimensional body and another screw inserted into a second angledscrew hole pierces the lower surface of the body when the plate iscoupled to the three-dimensional body.
 6. The system of claim 1 whereinthe axis of at least one of the angled screw holes extends toward theupper surface of the three-dimensional body when the front plate issecured to the three-dimensional body and the axis of at least one ofthe angled screw holes extends toward the lower surface of thethree-dimensional body when the front plate is secured to thethree-dimensional body.
 7. The system of claim 1 wherein the front plateis formed of a metallic material and the three dimensional body isformed from a non-metallic material.
 8. The system of claim 7 whereinthe three-dimensional body is made from a plastic material.
 9. Thesystem of claim 1 wherein the front plate has a superior surface and aninferior surface and a height between the superior surface and theinferior surface and wherein the front surface of the three-dimensionalbody has a height between the upper surface and lower surface that issubstantially equal to the height of the front plate.
 10. The system ofclaim 10 wherein the front surface of the three-dimensional body has aheight between the upper surface and the lower surface and the frontplate has a superior surface and an inferior surface and a heightbetween the superior surface and the inferior surface of the front plateis less than the height of the front surface of the three-dimensionalbody.
 11. The system of claim 1 wherein the first plate has a superiorsurface and an inferior surface and a height between the superiorsurface and the inferior surface and the three-dimensional body has aheight between the upper surface and lower surface, wherein the maximumheight of the three-dimensional body is substantially equal to orgreater than the height of the front plate.
 12. The system of claim 1wherein the three-dimensional body further includes at least one recessin communication with the front surface and the upper surface and atleast one recess in communication with the front surface and the lowersurface and the front plate is couplable with the three dimensional bodyso that at least one of the angled screw holes formed in the front platealigns with the at least one recess in communication with the uppersurface of the body and at least one of the angled screw holes formed inthe front plate aligns with the at least one recess in communicationwith the under surface of the body so that one of the bone screws isinsertable through one of the angled screw holes formed in the plate,through one of the plurality of recesses formed in the three dimensionalbodies and into one of the vertebrae.
 13. The system of claim 1 whereinthe three dimensional body includes at least one throughhole extendingfrom the upper surface to the lower surface.
 14. The system of claim 1wherein at least one of the angled screw holes formed in the plateincludes internal threads, and at least a portion of the head of one ofthe screws has external threads for engaging the internal thread of theangled screw hole.
 15. The system of claim 14 wherein the front plateincludes a plurality of angled screw holes having internal threads, anda plurality of screws having at least a portion of their headsexternally threaded and wherein the threaded portion of the angled screwholes are conically tapered.
 16. The system of claim 2 wherein thesecuring plate is fastenable parallel to the front plate by at least oneof a screw connection, a bayonet catch or a click catch.
 17. A spinestabilizing system for stabilizing first and second vertebrae in aspinal column comprising: a plurality of bone screws having a threadedshaft and a head, the shaft having a longitudinal central axis; a threedimensional body formed of plastic configured for insertion betweenendplates of the first and second vertebrae, the three dimensional bodyhaving an upper surface for abutting the endplate of a first vertebrae,a lower surface for abutting the endplate of a second vertebrae, and afront surface; and a front plate formed of a metallic material having atop surface, a bottom surface, a superior surface and an inferiorsurface, the plate couplable to the three dimensional body such that thebottom surface overlies the front surface, the plate having a pluralityof screw holes for receiving therethrough the shaft of the screws, thescrew holes having a central axis, at least two of the plurality ofscrew holes extending through the front plate so that the axis of thescrew hole is at an angle in the range of about 30° to about 70°relative to the bottom surface of the plate, wherein a first angledscrew hole is angled in the front plate toward the superior surface ofthe front plate and the second angled screw hole is angled in the frontplate toward the inferior surface of the front plate.
 18. The system ofclaim 17 further comprising a securing plate operatively connectable tothe front plate in such a manner to at least partially cover at leastone of the angled screw holes.
 19. The system of claim 17 wherein atleast one of the angled screw holes includes internal threads.
 20. Thesystem of claim 17 wherein at least one of the screws includes externalthreading at least partially on its head.
 21. The system of claim 17wherein the height of the front plate is substantially equal to theheight of the front surface of the three-dimensional body.
 22. Thesystem of claim 17 wherein the three dimensional body has a horizontalmiddle plane between the upper surface and the lower surface, and thefront plate is coupleable to the three dimensional body such that theaxis of at least two angled screw holes define an angle β ranging fromabout 20° to about 60° with the horizontal middle plane.
 23. The systemof claim 17 wherein the three dimensional body has a vertical middleplane extending from the front surface to a back surface and the frontplate is coupleable to the three dimensional body such that the axis ofthe at least two angled screws define an angle α ranging from about 10°to about 45° with the horizontal middle plane.
 24. The system of claim22 wherein the three-dimensional body has a vertical middle planeextending from the front surface to a back surface and the front plateis coupleable to the three-dimensional body such that the axis of the atleast two angled screws define an angle α ranging from about 10° toabout 45° with the horizontal middle plane.
 25. A spine stabilizingsystem for stabilizing first and second vertebrae in a spinal columncomprising: a plurality of bone screws having a threaded shaft and ahead, the shaft having a longitudinal central axis; a three dimensionalbody formed of plastic configured for insertion between endplates of thefirst and second vertebrae, the three dimensional body having a convexupper surface for abutting the endplate of a first vertebrae, a convexlower surface for abutting the endplate of a second vertebrae, a frontsurface, a back surface, a right side surface and a left side surface,the three dimensional body having a horizontal middle plane between theupper and lower surface and a vertical middle plane extending from thefront surface to the back surface; and a front plate formed of ametallic material having a top surface, a bottom surface, a superiorsurface and an inferior surface, the plate couplable to the threedimensional body such that the bottom surface overlies the frontsurface, the plate having a plurality of screw holes for receivingtherethrough the shaft of the screws, the screw holes having a centralaxis, at least two of the plurality of screw holes extending through thefront plate so that the axis of the screw hole is at an angle in therange of about 20° to about 60° relative to the horizontal middle planeof the body, wherein a first angled screw hole is angled in the frontplate toward the superior surface of the front plate and the secondangled screw hole is angled in the front plate toward the inferiorsurface of the front plate and wherein the height of the front platebetween the superior and inferior surfaces is substantially equal to orless than the maximum height between the upper and lower surfaces of thethree-dimensional body.